SUMMARY The overall goal of this project is to use the genetically tractable model organism C. elegans to dissect the molecular basis of axon regeneration after injury. The small size, transparent body, and simple anatomy of C. elegans allows single axons to be severed in vivo and their regrowth studied in depth. In the prior funding period we used large- scale genetic screens in C. elegans to discover conserved genes and pathways that play regrowth-promoting or regrowth-inhibiting roles in vivo. Many of these pathways are distinct from those involved in developmental axon outgrowth. Our large scale screens and analyses of genetic interactions have led to models for the function of these regrowth factors that we will test mechanistically in this proposal. We will dissect a signaling pathway that inhibits axon regrowth via axonal microtubule dynamics. We will investigate the role of membrane trafficking regulators in axon extension. Results from this work will elucidate intrinsic mechanisms that allow mature axons to regrow after damage. In vertebrates, peripheral nerves are capable of regrowth, yet recovery after peripheral nerve trauma is often slow and incomplete. The mammalian CNS undergoes minimal regeneration after injury, reflecting the combined effects of an inhibitory environment and of reduced intrinsic regrowth capacity. Improved knowledge of regrowth mechanisms will also inform our understanding of why CNS neurons do not regrow. Our work addresses intrinsic mechanisms that promote or inhibit axon regrowth, a high priority for this field. Many signaling pathways have conserved roles in axon regrowth, suggesting analysis of C. elegans axon regrowth has implications for understanding axon repair mechanisms in medically relevant situations.